The overall goal of this proposal is to develop a more detailed understanding of the molecular basis of biological electron transfer. Electron transfer is of central importance in all biological redox reactions and in energy conversion in the mitochondria and chloroplast, but the role that protein and cofactor structural changes play in these processes is just beginning.to be,elucidated. The spectroscopic studies to be described here will identify structural changes that accompany function in an electron transfer protein; the model system to be employed is the photosynthetic oxygen evolving complex, photosystem II. The factors that influence the efficiency of electron transfer after light absorption are likely to be important in other redox reactions, and photosynthetic proteins provide a unique and useful model system, since the reactions can be controlled by light. Moreover, photosystem II contains three redox active tyrosine residues. Therefore, photosystem Il provides an excellent system for studies of the factors that control the function of redox active amino acids. Redox active amino acids are now known to be present in-several enzymes, including prostaglandin H synthase, ribonucleotide reductase, galactose oxidase, and cytochrome c peroxidase. In photosystem II, one of the redox active tyrosines, Z, is oxidized by the primary donor and is reduced by the manganese cluster, which is the catalytic site of water oxidation. A second tyrosine, D, forms a stable radical of unknown function. The third redox active tyrosine, M, was discovered in my laboratory during the previous grant period, and its role in electron transfer is still under investigation. In this proposal, I describe experiments that bill elucidate the environmental factors that control the function of these redox active tyrosines. The experiments will utilize the spectroscopic techniques of EPR and reaction induced (difference) infrared in conjunction with isotopic labeling and site directed mutagenesis. There are three specific aims: (A) to investigate the protein factors that control the function of the redox active tyrosines, D and Z; (B) to investigate the structure, location, and kinetics of redox active tyrosine, M; (C) to isolate mutants that can be used for isotopic labeling of amino acids and prosthetic groups, such as chlorophyll and plastoquinone.